I want to know, if a String is a collection. I have read around but am still quite confused.
Strings are immutable objects representing a character sequence (CharSequence is one of the interfaces implemented by String).
Main difference to char arrays and collections of chars: String cannot be modified, it's not possible (ignoring reflection) to add/remove/replace characters.
Internally they are represented by a char array with an offset and length (this allows to create lightweight substrings, using the same char arrays).
Example: ['F','o','o','H','e','l','l','o',' ','W','o','r','l','d'], offset=3, count=5 = "Hello".
strings are object of class String and it's not a Collection as well. It is implemented on top of char array. You can see it in the below code snippet:
public final class String implements
java.io.Serializable, Comparable<String>, CharSequence
{
private final char value[];
No, it's not a collection in that it does not implement the Collection<E> interface.
Conceptually, it is an immutable sequence of characters (and implements the CharSequence interface).
Internally, the String class is likely to use an array of chars, although I am pretty sure the spec does not mandate that.
No, A string is an object of class String
The String class represents character strings. All string literals in
Java programs, such as "abc", are implemented as instances of this
class....
A String represents a string in the UTF-16 format in which
supplementary characters are represented by surrogate pairs (see the
section Unicode Character Representations in the Character class for
more information). Index values refer to char code units, so a
supplementary character uses two positions in a String.
No it's not an Array or a Collection. However there is a convenient method if you need a char array and you have a String - namely String.toCharArray(); you could use it like this
// Prints the String "Hello, World!" as a char[].
System.out.println(Arrays.toString("Hello, World!".toCharArray()));
No, a String is an immutable string of characters, and the class extends Object directly and does not implement any of the Collection interfaces. Really, that's the basic answer.
However, there's a lot going on under the covers in the runtime--there's a whole collection of cached strings held by the JVM and in its most primitive representation, yeah, a String is basically an array of characters (meaning it's a bunch of memory addresses pointing to representations of characters). Still, once you go below the definition of String as it's defined as a class, you can keep going until you get to the point that you're just talking about organized combinations of ones and zeroes.
I'm guessing here, but I imagine you posed the question because you're studying this and an instructor said something about a String being an array of characters. While technically correct, that's really confusing because the two concepts exists at completely different levels of abstraction.
String Class is just a class which allows you to create objects of strings. Like all classes it makes easy the use of these objects using its owns methods.
An array of char is an string, but its methods are relative to arrays or the elements of the array.
So for example if you want to look for the position of a carácter in an string of a class String then you use a method called IndexOf.
But if you want to find a character in an array of char, then you would have to do it manually using loops.
Related
I am currently reading a textbook on Java and each chapter involving the String datatype also discusses char. My question is, what purpose does char have in the real world?
The only thing I have found is that because String is immutable, it makes it a poor choice for passwords. Thus, one should choose a character array (char[]) over String. However, Java does have a mutable class for strings called StringBuilder; would that not be just as suitable a replacement for strings as is char[]?
This has already been answered in the comments really :)
A String is a collection of Chars. Without Chars you would have nothing to build Strings from.
Because Char[] and dealing with Char[] is so important they warrant having their own class to handle the processing of them - hence String.
In your coding you are unlikely to use the Char datatype directly unless you are processing Strings or handling passwords. The only reason Char[] is used for passwords is because it's harder to accidentally print them into logs/view them in memory/put them into string caches, etc and because once you have finished with it you can explicitly zero the elements in the Array so it never stays in memory longer than needed.
In C++ I can do something like this...
String s = "abc";
char c = s[i]; // works fine...
But in Java, if I try doing the same, it throws an error. Why?.
In java, to achieve the above, I have to do :
s.toCharArray();
How is the implementation of Strings in C++ different from that in Java?
In java, to achieve the above, I have to do :
s.toCharArray();
Not really. You can use charAt instead:
char c = s.charAt(i);
Basically, C++ allows user-defined operators - Java doesn't. So the String class doesn't expose any sort of "indexing" operator; that only exists for arrays, and a String isn't an array. (It's usually implemented using an array, but that's a different matter.)
EDIT: As noted in comments, the + operator is special-cased for strings - right in the language specification. The same could have been done for [], but it isn't - and as it's not in the language specification, and Java doesn't support overloaded operators, it can't be performed in library code. (For example, you can't give custom behaviour to + for any other class.)
The difference is that C++ has operator overloading, and uses it to access the string contents.
They both store the string characters in such a way as you cannot change them.
The reason that it is possible to write
string s = "abc";
char c = s[i];
in C++ is that the string class has overloaded the indexing operator (say [] operator) which allows programmers to access characters of a string object the same way that they access an element of an array, despite the fact that a string object is not an array.
Java, on the other hand, does not allow operator overloading of any kind (the only exception is the + operator that is overloaded for strings) and hence, the indexing operator is not and can not be overloaded for String objects. In Java, to access a character of a string, you need to use accessor member methods, such as charAt. You can also invoke the toCharArray method of the String class, which returns to you an array of the characters of the string object and you can use the indexing operator with this returned value:
char c = s.toCharArray()[i];
See the method String#charAt
Returns the char value at the specified index. An index ranges from 0 to length() - 1. The first char value of the sequence is at index 0, the next at index 1, and so on, as for array indexing.
If the char value specified by the index is a surrogate, the surrogate value is returned.
public char charAt(int index)
in c++ strings are already treated as array of characters,
but in java String is a built in class.
it is different from array of characters.
In C++, a string is typically just an array of (or a pointer to) chars, terminated with a NULL (\0) character. You can process a string by indexing also as you would process any array.
But in Java , a strings are not arrays. Java strings are objects of type java.lang.String so You cannot process them by indexing .
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I am learning Java this year as part of the AP Computer Science curriculum, and while I was reading about "Char" and "String" I could not understand why one would bother to use "Char" and only be able to store one character rather than just use "String" and be able to store much more than that. In short what's the point of "char" if it can only store a single character?
People are mentioning memory concerns, which are valid, but I don't think that's a very important reason 99% of the time. An important reason is that the Java compiler will tell you if you make a mistake so you don't have to figure it out on your own.
For example, if you only want 1 character for a variable, you can use a char to store the value and now nobody can put anything else in there without it being an error. If you used a String instead, there could be two characters in the String even though you intended that to never be possible. In fact, there could be 0 characters in the String which would be just as bad. Also, all your code that uses the String will have to say "get the first character of the String" where it could simply say, "give me the character".
An analogy (which may not make sense to you yet, unfortunately) would be, "Why would I say a Person has a Name when I could say a Person has a List of Names?" The same reasons apply. If you only want a Person to have one Name, then giving him a list of Names adds a lot maintenance overhead.
You could consider this analogy:
You need one apple. Would you prefer to have one apple in your hand, or a big box that could contain more apples, but only needs to contain the one?
The char primitive datatype is easier to work with than the String class in situations where you only need one character. It's also a lot less overhead, because the String class has a lot of extra methods and information that it needs to store to be efficient at handling string with multiple characters. Using the String class when you only need one character is basically overkill. If you want to read from a variable of both types to get the character, this is the code that would do that:
// initialization of the variables
char character = 'a';
String string = "a";
// writing a method that returns the character
char getChar()
{
return character; // simple!
}
char getCharFromString()
{
return string.charAt(0); // complicated, could fail if there is no character
}
If this code looks complicated, you can ignore it. The conclusion is that using String when you only need one character is overcomplicating things.
Basically, the String class is used when you need more than one character. You could also just create an array of chars, but then you would not have the useful methods of the String class, such as the .equals() and .length() methods.
Strings are objects. Objects always go on the dynamic storage. Storing one-character string would require at least a dozen of bytes.
chars (not Chars) are primitives. They take fixed amount of space (2 bytes). In situations when you need to process a single character, creating one-character string is a waste of resources. Moreover, when you expect to see a single character, using strings would require validation that the data passed in has exactly one character. This would be unacceptable in situations when you must be extremely fast, such as character-based input and output.
To summarize, you need a char because of
Memory footprint - a char is smaller than a String of one character
Speed of processing - creating objects carries an overhead
Program's maintainability - Knowing the type makes it easier for you and for the readers of your code to know what kind of data is expected to be stored in a char variable.
char take up less memory for times when you really only need one character. There are also multiple other applications for using a single character.
char is a primitive datatype while string is an object which comes at greater overhead.
A string is also made up of char, so there's that too.
Because the char takes up less memory!
Also the char is stored in memory and NOT as a reference value so theoretically its faster to access the char (You'll understand that more later)
***Note: I once had this same thought when I first started programming about why use an int when you can use a long and not have to worry about large numbers. This tells me you're on your way to be a great programmer! :)
char is a primitive type while String is a true Object. In some cases where performance is a concern it's conceivable that you would only want to use primitives.
Another case in which you would want to use char is when you're writing Java 1.0 and you're tasked with creating the String class!
public final class String
implements java.io.Serializable, Comparable<String>, CharSequence {
/** The value is used for character storage. */
private final char value[];
Everything in java can be reduced to primitive types. You can write any program with primitive types. So you need some kind of minimalist way of storing text. A char is also really just a byte, that is interpreted as a character.
Also if you want to loop though all characters in a string you would do:
char[] chArr = str.toCharArray();
for(int i = 0 ; i < chArr.length ; i++)
{
//do something with chArr[i];
}
This would be much more awkward trying to substring out an exact character from the String.
Lot of answers here already. While the memory concerns are valid, you have to realize there are times when you want to directly manipulate characters. The word ladder game
where you try to turn one word into another by changing one character at a time is an example I had to do in a programming class. Having a char type lets you manipulate a singe character at a time. It also lets you assign an int to a char that maps to your local character set.
You can do thing like char c = 97; and that will print out as a. You can do things like increment a character from 97 to 122 to print out all lowercase characters. Sometimes this actually is useful.
I have some question that I wonder about. I know that string are immutable in Java and therefore a new string object is created rather than changed when for example assigning to a existing string object.
Now to my question. Let's suppose that I have the following piece of code:
String a = "Hello World";
String b = "Hello World";
String res = a.substring(0,4) + b.substring(6,10);
How many string objects will be created by the code at line 3 ? Will each call to substring create a new string object ? Will my code above generate 3 new string objects ?
Thanks in advance
Strings in Java are immutable. Basically this means that, once you create a string object, you won't be able to modify/change the content of a string. As a result, if you perform any manipulation on a string object which "appears to" change the content of the string, Java creates a new string object, and performs the manipulation on the newly created one.
Based on this, your code above appears to create five string objects - two are created by the declaration, two are created by calls to substring, and the last one is created after you concatenate the two pieces.
Immutability however leads to another interesting consequence. JVM internally maintains something like a string pool for creating string literals. For saving up memory, JVM will try to use string objects from this pool. Whenever you create a new string literal, JVM will loop into the pool to see if any existing strings can be used. If there is, JVM will simply use it and return it.
So, technically, before Java 7, JVM will create only one string object for your whole code. Even your substring calls won't create new string objects in the pool, it will use the existing "Hello World" one, but in this case it will only use characters from position 0 to 3 for your first call to substring, for example. Starting from Java 7, substring will not share the characters, but will create a new one. So, total object count will be 4 - the last one will be created with the concatenation of the two substrings.
Edit
To answer your question in the comment, take a look at Java Language Specification -
In the Java programming language, unlike C, an array of char is not a
String, and neither a String nor an array of char is terminated by
'\u0000' (the NUL character).
A String object is immutable, that is, its contents never change,
while an array of char has mutable elements.
The method toCharArray in class String returns an array of characters
containing the same character sequence as a String. The class
StringBuffer implements useful methods on mutable arrays of
characters.
So, no, char arrays are not immutable in Java, they are mutable.
Literal a is created newly and kept in the pool.Literal b refer the a, it will not create new one instead.
The line 3 will create 3 new String since substring creates a new string and concatenate creates new Strings every time.
String substring(int beginIndex,int endIndex)
Returns a new string that is a substring of this string. The substring
begins at the specified beginIndex and extends to the character at
index endIndex - 1. Thus the length of the substring is
endIndex-beginIndex.
In my program I'm trying to compare my char array asterixA[] to a String word in an if condition like:
if (word.equals(asterixA))
but it's giving me an error. Is there any other way I can compare them?
you have to convert the character array into String or String to char array and then do the comparision.
if (word.equals(new String(asterixA)))
or
if(Arrays.equals(word.toCharArray(), asterixA))
BTW. if is a conditional statement not a loop
You seem to be taking the "A String is an array of chars" line too literal. String's equals method states that
Compares this string to the specified object. The result is true if and only if the argument is not null and is a String object that represents the same sequence of characters as this object.
It all depends of the circumstances, but generally you compare two objects of the same type or two objects belonging to the same hierarchy (sharing a common superclass).
In this case a String is not a char[], but Java provides mechanisms to go from one to the other, either by doing a String -> char[] transformation with String#toCharArray() or a char[] -> String transformation by passing the char[] as a parameter to String's constructor.
This way you can compare both objects after either turning your String into a char[] or vice-versa.
You can compare the arrays:
if (Arrays.equals(asterixA, word.toCharArray()) {}
do as follows: if (word.equals(new String(asterixA))) { ... }